1. Field of the Invention
This invention is directed to rotation sensors, and more particularly to a rotation sensor having a pulse transducer, seated on a rotation element, in particular on a shaft of a motor drive mechanism.
2. Description of the Prior Art
The term rotation sensor will be used hereinafter as a synonym for an rpm or rotation angle meter. Such rotation sensors are most often made using stationary Hall sensors or rotating ring magnets that have north and south poles alternating in the circumferential direction. Rotational angles can be measured directly with analog Hall sensors, and rotational speeds, i.e., rpm, can be measured directly with digital Hall sensors. From the rpm or rotation angle, further measurement variables can be derived in turn, such as the rotary travel or stroke travel.
A known rotation sensor of this type disclosed in European Patent Disclosure EP 0 821 240 A1 is disposed in a roller bearing, and the pulse transducer is seated on the inner ring of the bearing that is connected to a shaft in a manner fixed against relative rotation, and the pickup that contactlessly senses the pulses is secured to the stationary outer ring of the roller bearing. The ring magnet, made of a plastoferrite, of the pulse transducer has a hollow magnet body with successive north and south poles on the circumference, and upon its rotation it generates a change in the magnetic field relative to a fixed point. These magnetic field changes are converted into electrical signal by stationary pickups. For fastening the pulse transducer to the inner ring of the roller bearing, the ring magnet is seated in a manner fixed against relative rotation on the metal bush, embodied as a thin sheet-metal sleeve, and this fastening is accomplished by crimping over the edge of the sleeve onto the annular face end of the hollow-cylindrical magnet body. The sheet-metal sleeve is press-fitted in turn onto the outer surface of the inner ring of the roller bearing.
In another known rotation sensor of the type defined at the outset (EP 0 601 228 B1), the ring magnet of the pulse transducer is made from magnetizable particles, which are available in powder form, and a binder of thermoplastic material. The proportion of powder comprising magnetizable particles is selected to be as high as possible, so that the ring magnet molded by injection molding will be capable of tripping the clearest possible signals in the pickup. However, this makes the magnet body brittle, and the magnet body is therefore given an inside diameter that in combination with the diameter of the shaft of a drive motor produces a sliding seat and the magnet body is glued or bonded to the shaft. One disadvantage is that the adhesive contains solvents and/or other chemicals and requires a long time to set.
To avoid gluing the hollow-cylindrical magnet body to the shaft, EP 0 601 228 B1 has already proposed using a retaining part, which when paired with the shaft forms a press fit, for transmitting torque from the shaft to the hollow-cylindrical magnet body. This transmission is done by means, such as arms, that act by positive engagement and have hooks protruding axially from the free ends and extend through grooves provided in the hollow-cylindrical magnet body. This retaining part is injection molded integrally with the arms and hooks from thermoplastic material.
The rotation sensor according to the present invention has the advantage that the production problems that occur in gluing are avoided, and the pulse transducer can be made in a way that is simple from a production standpoint. The press fit assures the most accurate possible alignment of the ring magnet with the rotation element, and the metal bush assures a reliable connection, in a manner fixed against relative rotation, of the ring magnet to the rotation element, and this connection can be made in a manner that is favorable from a production standpoint in the bush portion that protrudes from the ring magnet. Preferably, a clearance fit is provided between the rotation element, embodied for instance as a supported shaft, and the metal bush, so that in assembly, the bearing point on the shaft will not be damaged. If the entire rotation sensor does not have to be thrust over a bearing point, then a transitional or press fit can be used.
The rigid connection of the bush to the rotation element inside the bush portion that protrudes from the ring magnet and the embodiment of the shaped elements on the circumference of the metal bush can be accomplished in various ways.
In one preferred embodiment of the invention, the fastening of the bush portion to the rotation element is done by press resistance welding, known as hot staking, in which both a tensing force and an electric current are applied to the bush portion by means of electrodes radially positioned against it.
In further advantageous embodiments of the invention, the connection between the bush portion and the rotation element can also be made by nonpositive pressing of the bush portion on the rotation element or by positive engagement between the bush portion and the rotation element.
In an advantageous embodiment of the invention, the shaped elements on the circumference of the metal bush for establishing the positive engagement between it and the ring magnet are embodied as knurling, axial grooves, a set of notched teeth, and the like.